Classical stochastic view of cell growth and differentiation insists on a specific progenitor cell origin, a process of growth resulting in a mature (terminally differentiated) cell which cannot further divide. In the liver, a pluripotential stem (oval) cell gives rise to both hepatocytes and biliary ductal cells. A bone marrow derived progenitor cell also contributes to the growth and differentiation of liver and biliary ductal cells. Recent emphasis has focused on the plasticity of stem cells. The concept of 'plasticity' of the well differentiated cell has been largely ignored. This proposal examines the provocative hypothesis that it is possible to have transdifferentiation between the mature cells of the hepatocyte and biliary epithelial cell lineages. Mature hepatocytes and biliary duct epithelia divide actively in vivo and in vitro. Culture conditions in vitro (growth factors, cytokines, extracellular matrix) can induce phenotypic transdifferentiation of hepatocytes into biliary cells and vice versa. Experiments are designed to (1) further characterize a biliary cell line which we have established from the MT-lacZ mouse, with a transgene encoding Beta-galactosidase (X-gal) driven by an albumin promoter and hence only hepatocytes will stain blue. (2) induce transdifferentiation of biliary cells into hepatocytes in vitro, Cells will be studied with lineage-specific and maturation-dependant markers, albumin secretion, and X-gal positivity. The potential role of Notch gene in regulating cell lineage decision will be studied. Notch pathway gene expression during cell growth and differentiation will be determined using semi-quantitative RT-PCR. In addition, the functional role in cell lineage commitment and differentiation will be studied by inducing Notch signalling using exogenous Notch ligands (Jagged 1, Delta 1). (3) study the in vivo propagation of transdifferentiated cells by implanting them into the omental compartment of congenic mice, and (4) to use transdifferentiated cells of biliary origin in cell transplantation to repopulate and rescue the dying livers of Albu-PA mice (carrying a cytotoxic transgene resulting in liver failure). The results of these experiments will yield new, important information on cell lineage specificity, regulation of cell commitment and cell fate, and the effect of instructive transdifferentiation. It potentially can open up a new field of using mature cells of the same embryonic origin to mediate gene.